US 7492814 B1 Abstract Removing noise and interference from a signal by calculating a joint time-frequency domain of the signal, estimating instantaneous frequencies of the joint time-frequency domain, modifying each estimated instantaneous frequency, if necessary, to correspond to a frequency of the joint time-frequency domain to which it most closely compares, redistributing elements within the joint time-frequency domain according to the modified instantaneous frequencies, computing a magnitude for each element in the redistributed joint time-frequency domain, plotting the results, identifying peak values, eliminating from the redistributed joint time-frequency domain elements that do not correspond to the peak values, identifying noise and interference in the peak values, eliminating the noise and the interference from the redistributed joint time-frequency domain elements, and recovering a signal devoid of noise and interference from the modified redistributed joint time-frequency domain.
Claims(12) 1. A method of removing noise and interference from a signal, comprising the steps of:
a) receiving the signal;
b) calculating a joint time-frequency domain of the received signal, where the joint time-frequency domain of the received signal includes elements that represent the received signal;
c) estimating instantaneous frequencies of the joint time-frequency domain;
d) modifying each estimated instantaneous frequency, if necessary, to correspond to a frequency of the joint time-frequency domain to which it most closely compares;
e) redistributing the elements within the joint time-frequency domain according to the estimated instantaneous frequencies as modified; and
f) computing a magnitude for each element in the joint time-frequency domain as redistributed;
g) plotting the results of the step (f) as the time-frequency representation of the received signal;
h) identifying peak values in the plot of step (g);
i) eliminating from the redistributed joint time-frequency domain elements that do not correspond to the peak values identified in step (h);
j) identifying noise and interference in the peak values identified in step (h);
k) eliminating from the redistributed joint time-frequency domain elements that correspond to the noise and the interference identified in step (j); and
l) recovering a signal devoid of the noise and the interference from the redistributed joint time-frequency domain as modified in step (k).
2. The method of
3. The method of
4. The method of
5. The method of
6. The method of
(a) determining arguments for each element in the short-time Fourier Transform matrix;
(b) forming an argument matrix from the results of step (a), where each element in the argument matrix corresponds to the element in the short-time Fourier Transform matrix from which the argument was determined;
(c) calculating a derivative of the argument matrix; and
(d) forming an instantaneous frequency matrix from the results of step (c), where each element in the instantaneous frequency matrix corresponds to the element in the argument matrix from which the instantaneous frequency matrix element was derived.
7. The method of
8. The method of
9. The method of
(a) identifying, for each element in the short-time Fourier Transform, the instantaneous frequency that corresponds position-wise to the element in the short-time Fourier Transform;
(b) identifying the value of the identified instantaneous frequency; and
(c) moving the corresponding element in the short-time Fourier Transform to a location within its matrix column that corresponds to the identified value of the corresponding instantaneous frequency, summing all of the short-time Fourier Transform elements that map to a same location.
10. The method of
(a) identifying elements in the magnitudes computed in step (f) that are not associated with the peak values; and
(b) eliminating the elements in the redistributed joint time-frequency domain that do not correspond to the identified magnitudes.
11. The method of
(a) identifying elements in the magnitude computed in step (f) that are associated with the peak values that are below a user-definable threshold; and
(b) identifying elements in the magnitudes computed in step (f) that are associated with the peak values that exhibit period signal behavior and are empirically determined to be the interference.
12. The method of
(a) summing the matrix axis that represents discrete time in the redistributed joint time-frequency domain as modified by step (i); and
(b) listing results of step (b), in sequence, as the received signal devoid of the noise and the interference.
Description The present invention relates, in general, to speech signal processing and, in particular, to noise and interference reduction. A frequently recurring problem in communications is the need to remove noise and interference from a signal. There have been many approaches suggested to address this problem. For stationary narrowband interference, a simple notch filter may be effective. Adaptive notch filters have been suggested to remove narrowband non-stationary interference. For removal of broadband noise from speech, Wiener filter techniques such as spectral subtraction are frequently used. An adaptive filter can be used to remove single non-stationary narrowband interfering signal from a broadband, or speech, signal. In this process, the frequency of the interfering signal is estimated at each time, and a single estimate of the interfering signal bandwidth is estimated. The received signal is frequency shifted at each time such that the frequency of the interfering signal is constant in the shifted representation. A fixed notch filter is applied to remove the interfering signal, and the filtered signal is frequency shifted to its original frequency. This process can remove a single interfering narrowband signal and it can, in principle, be iterated to remove several interfering signals. However, the primary problem with using an adaptive filter is that the instantaneous frequency of the interference must be accurately estimated at each time. Such estimation is difficult. Furthermore, an adaptive filter cannot be used to remove noise. In Wiener filtering, or spectral subtraction, components which are dominated by noise are removed from the spectrogram, and the noise-free, or clean, signal is estimated by a pseudo inversion process. Spectral subtraction is the most common enhancement method used in speech processing. In this method, a short time Fourier transform (STFT) is computed. The STFT is a time-frequency representation. STFT components identified as dominated by noise are reduced in magnitude, and a signal whose STFT approximates the modified STFT is computed. The clean signal estimated this way is only an approximate solution because the modified STFT cannot be inverted. In the pseudo inversion process, some ad hoc criterion must be used. Furthermore, spectral subtraction cannot be easily used to remove narrowband interference. For stationary narrowband interference, a clean signal can be estimated by using a notch filter which is tuned to the interference frequency and bandwidth. A notch filter cannot remove noise or interference whose frequencies change with time. None of the prior art methods adequately identify and separate signal and non-stationary interference components. U.S. Pat. No. 6,175,602, entitled “SIGNAL NOISE REDUCTION BY SPECTRAL SUBTRACTION USING LINER CONVOLUTION AND CASUAL FILTERING,” discloses a method of reducing noise in a signal by spectral subtraction. The present invention does not use spectral subtraction as does U.S. Pat. No. 6,175,602. U.S. Pat. No. 6,175,602 is hereby incorporated by reference into the specification of the present invention. U.S. Pat. No. 6,266,633, entitled “NOISE SUPPRESSION AND CHANNEL EQUALIZATION PREPROCESSOR FOR SPEECH AND SPEAKER RECOGNIZERS: METHOD AND APPARATUS,” discloses a device for and method of noise suppression that uses blind deconvolution. The present invention does not use blind deconvolution as does U.S. Pat. No. 6,266,633, U.S. Pat. No. 6,266,633 is hereby incorporated by reference into the specification of the present invention. U.S. Pat. No. 6,795,559, entitled “IMPULSE NOISE REDUCER DETECTING IMPULSE NOISE FROM AN AUDIO SIGNAL,” discloses a method of reducing noise by detecting and smoothing the high frequency amplitude of a signal, attenuating the non-smoothed amplitude of the signal, comparing the attenuated amplitude to a threshold, and identifying and removing the noise. The present invention does not smooth the high frequency amplitude of a signal as does U.S. Pat. No. 6,795,559. U.S. Pat. No. 6,795,559 is hereby incorporated by reference into the specification of the present invention. U.S. Pat. No. 6,801,889, entitled “TIME-DOMAIN NOISE SUPPRESSION,” discloses a method of reducing noise by performing a Fourier Transform on the signal to generate a frequency spectrum, performing an Inverse Fourier Transform to simulate a noise signal, and subtracting the simulated noise signal from the time-domain signal. The present invention does not simulate noise using an Inverse Fourier Transform as does U.S. Pat. No. 6,801,889. U.S. Pat. No. 6,801,889 is hereby incorporated by reference into the specification of the present invention. U.S. Pat. No. 6,826,392, entitled “MULTIPATH NOISE REDUCTION METHOD, MULTIPATH NOISE REDUCER, AND FM RECEIVER,” discloses a method of reducing noise by frequency modulating the signal, extracting a high-frequency signal from the modulated signal, generating a noise reduction coefficient from the extracted high-frequency signal, separating the signal into high-frequency and low frequency components, multiplying the high-frequency component by the noise reduction coefficient, and adding the product to the low-frequency component. The present invention does not generate a noise reduction coefficient as does U.S. Pat. No. 6,826,392. U.S. Pat. No. 6,826,392 is hereby incorporated by reference into the specification of the present invention. U.S. patent application No. 20020173276 A1, entitled “METHOD FOR SUPPRESSING SPURIOUS NOISE IN A SIGNAL FIELD,” discloses a method of reducing noise by determining a distribution function of the signal, comparing the distribution to a reference distribution, modifying the components in the signal that differ from the reference distribution, and not modifying the components in the signal that are the same as those in the reference distribution. The present invention does not use a reference distribution as does U.S. patent application No. 20020173276 A1 U.S. patent application No. 20020173276 A1 is hereby incorporated by reference into the specification of the present invention. It is an object of the present invention to remove noise and interference from a speech signal. It is another object of the present invention to remove noise and interference from a speech signal by concentrating a short time Fourier transform (STFT) and estimating a noise-free and interference-free signal by integration. It is another object of the present invention to remove noise and interference from a speech signal by summing a user-definable set of peak values in the concentrated STFT. The present invention is a method of removing noise and interference from a speech signal by concentrating a STFT and estimating a noise-free and interference-free signal by integration. The first step of the method is receiving the signal. The second step of the method is converting the received signal to the joint time-frequency domain. The third step of the method is estimating an instantaneous frequency (IF) for each element in the joint time-frequency domain calculated in the second step. The fourth step of the method is modifying each result of the third step, if necessary, where each IF element is replaced, if necessary, with the discrete frequency of the joint time-frequency domain created in the second step to which it most closely compares in value. The fifth step of the method is redistributing the elements within the joint time-frequency domain created in the second step according to the IF elements as modified by the fourth step. The sixth step of the method is computing, for each time, the magnitude of each element of the joint time-frequency domain as redistributed in the fifth step. The seventh step of the method is plotting the results of the sixth step in a graph as the time-frequency representation of the received signal. The eighth step of the method is identifying in the plot of the seventh step the maximum, or peak, values. The ninth step of the method is eliminating from the redistributed joint time-frequency domain those elements that do not correspond to the peak values identified in the eighth step. The tenth of the method is identifying noise and interference in the result of the eighth step. The eleventh step of the method is eliminating from the redistributed joint time-frequency domain those elements that correspond to the noise and interference identified in the tenth step. The twelfth, and last, step of the method is recovering a signal devoid of noise and interference from the redistributed joint time-frequency domain as modified in the eleventh step The present invention is a method of removing noise and interference from a signal. The present method is useful in signal coding application, but may be used for any other suitable signal processing application. The first step The second step
The third step The fourth step The fifth step
The result of the fifth step 5 is a novel time-frequency representation. When applied to a multi-component signal which has linearly independent components and which are separable, the method produces a time-frequency representation in which the value of each signal component is distributed, or concentrated, along the component's instantaneous frequency curve in the time-frequency plane. The concentrated STFT is a linear representation, free of cross-terms, which plagued the prior art methods, and having the property that signal and interference components are easily recognized because their distributions are more concentrated in time and frequency. A plot of the remapped matrix is necessary to see that the elements have been so remapped. The following steps result in such a plot.
The sixth step The seventh step The eighth step The ninth step The tenth step The eleventh step The twelfth, and last, step Patent Citations
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